1. Field of the Invention
The present invention is directed to power conversion and more particularly to generation of VCC used in an integrated circuit forming part of the power conversion system.
2. Description of the Related Art
Switching voltage regulators are used to provide, e.g., regulated DC output voltage from an unregulated AC input. Typical consumer products involving such switching regulators include cell phone chargers, laptop or printer power supplies (so-called “bricks”), and embedded PC power supplies.
FIG. 1 illustrates switching regulator 100 having a topology commonly used in power supplies. The switching regulator includes high voltage isolation between the DC output (secondary side) and the AC input (primary side), which is required for safety/regulatory reasons and may also be required for functional reasons. The power is transferred from the primary side to the secondary side using a transformer 102 and primary side high voltage switch 104.
The switching regulator shown in FIG. 1 operates as follows. When the power MOSFET switch 104 turns ON (TON phase) according to the gate control signal supplied by the controller integrated circuit 106, the current through primary winding ramps up with a slope of Vin/Lp and the energy stored in the transformer core at the end of the TON cycle is proportional to 0.5*Lp*Ippeak2, where Lp is the transformer primary winding inductance and Ippeak is the primary winding peak current. The output current Is is zero during the TON phase and the voltage Vs is negative referenced to the secondary side ground, Vs=−Ns*Vin, where Ns is the transformer secondary/primary turn ratio.
When switch 104 turns OFF (TOFF phase), the primary inductor current Ip becomes zero and secondary current Is ramps down from the value Ispeak=Ippeak/Ns to zero, with a slope of approximately ˜(Vout+Vdout)/Ls. Accurate regulation of the output voltage requires feedback proportional to output voltage. The feedback controls the duty cycle of switch 104 in order to keep the output voltage constant over changing load and input voltage. The feedback path needs to cross the isolation barrier between the primary and secondary. A common feedback solution uses an opto-coupler 108 as shown in FIG. 1.
Another aspect shown in FIG. 1 is the use of an auxiliary winding 110 to provide the power supply VCC to the integrated circuit 106, which supplies the gate control signal for transistor 104.